Electric controls with transistors



June 8,1965 s. A. ZARLENG- 3,188,501

ELECTRIC CONTROLS WITH TRANSISTORS Original Filed June 24, 1958 v 2Sheets-Sheet 1 INVENT OR.

June 8, 1965 s. A. 'ZARLENG 3,188,501

' ELECTRIC CONTROLS WITH TRANSISTORS Original Filed June 24; 1958 .2Sheets-Sheet 2 a hi INVENTOR.

572m 2? Zar/My United States Patent 3,188,501 ELECTRIC CONTROLS WITHTRANSISTORS Steve A. Zarleng, Akron, Ohio, assignor to The ClarkController Company, Cleveland, Ohio, a corporation of Ohio Originalapplication June 24, 1958, Ser. No. 744,251, now a Patent No. 3,070,710,dated Dec. 25, 1962. Divided and this application July 20, 1962, Ser.No. 211,453 6 Claims. (Cl. 307-112) This application is a division of mycopending application, Serial No. 744,251, filed June 24, 1958, nowPatent No. 3,070,710.

"The invention hereof relates to electric control systems; and relatesmore particularly to control systems or circuit complexes that operatein a manner to supply output current from a current source to a load tobe energized, alternately, at a high value, and at zero value.

Such control systems are known and are sometimes arranged to respond inthis manner to the operation and restoring of two contactors. When onecontactor is operated, it causes the system to deliver high output andto maintain that high output after the contactor has been restored; andwhen the other contactor is operated, it causes the system to reduce itsoutput to zero value, and to so maintain it when said other contactor isrestored.

The present invention relates more particularly to control systemshaving the aforesaid characteristics of operation.

In my copending patent application, Serial No. 507,- 643, now Patent No.2,966,979, I have shown in FIG- URE 1 a control system of this type, andthe present invention may be considered as an improvement thereover.

In the control system of that application, two contactors and threetransistors are utilized. Upon the operation of one contactor, a firstone of the transistors is rendered conducting, and is immediatelymaintained conducting by a second transistor, and current flows throughthe first transistor and to a third transistor, rendering it conducting,and current is conducted through the third transistor to a load.

The said one contactor may be restored without changing theseconditions.

Then the load current may be reduced to zero value by operation of theother one of the two contactors, which renders the first transistornon-conducting and it is maintained non-conducting when the said othercontactor is restored.

The present invention is shown and described hereinafter in two forms,each of which is an improvement over that of my said pending applicationin that, among other differences, it requires only one transistor.

In the first form, upon operating one of the two contactors thetransistor is rendered conducting, and current from a supply sourceflows at full value through the transistor and through the load; andcurrent is derived from the load current to actuate a feedbacktransformer, the output of which maintains the transistor conducting.The said one contactor may then be restored, Without changing theseconditions.

Then the load current may be reduced to zero value by operating theother contactor, which renders the transistor nonconducting.

As an alternative mode of operation of this form, after the onecontactor has been operated and the full value of current flows throughthe load as described, the one contactor may be maintained operated andnot restored; and to reduce the load current to zero value, the saidother contactor may be operated to render the transistor non-conducting.

Then to again cause full load current to fiow, the said other contactormay be restored. The said one contactor being still operated, thetransistor will be rendered conducting thereby, and full current to theload will again flow as described.

According to this alternative mode of operation, when the transistor hasonce been rendered conducting by operating the one contactor, and it isthen maintained operated, the transistor may be rendered non-conductingand again conducting, alternately, by alternately operat ing andrestoring the said other contactor.

In the second form of the invention a saturable transformer is providedhaving two opposed primaries energizable respectively by the aforesaidtwo contactors; and having a secondary connected to the transistor.

By one mode of operation, upon closing one contactor, one primarysaturates the transformer, and in doing so, a pulse of secondary currentrenders the transistor conducting and full value of load current flowsthrough it and current derived from the load current maintains itconducting. The one contactor may then be opened.

The load current may then be reduced to zero value, by closing the othercontactor to energize the other primary to cause it to saturate thetransformer in the opposite direction which causes a pulse of secondarycurrent to render the transistor nonconducting and stop the flow of loadcurrent through it. The said other contactor may then be opened.

According to an alternative mode of operation of this second form of theinvention, one primary is always energized to saturate the transformerin the direction to cause the transistor to be conducting, and the otherprimary has sufficient magnetomotive force to overpower that of the oneprimary and reverse the saturation of the transformer and render thetransistor nonconducting.

The load current is therefore caused to flow at full value or to be cutoff, respectively, by alternately opening and closing a single contactorthat effects de-energization and energization of the overpoweringprimary.

It is among the objects of the invention:

To provide, generally, an improved control the type referred to above;

To provide an improved control system having, others, features set forthin the foregoing tion of the invention;

To provide a control system comprising a transistor in the path of loadcurrent from a current source through a load and rendered conducting byinitiation of current flow in an activating current path in thetransistor in a system of among brief descripiirst direction andmaintained conducting by current in said activating path in saiddirection derived from the load current; and rendered non-conducting byinitiation of current in said activating path in the opposite direction.

Other objects will occur to those skilled in the art, from a reading ofthe following full description of the invention.

An embodiment of the invention and a modification thereof are fullydescribed in the following description taken in connection with theaccompanying drawings in which:

FIGURE 1 is a diagrammatic view of an electric system embodying theinvention in one form;

FIGURE 2 is a diagrammatic view of a modification of the embodiment ofFIGURE 1;

FIGURE 3 is a view illustrating a hysteresis curve, by reference towhich, operation of the modification of FIGURE 2 is explained; and

FIGURE 4 is a view of the embodiment of FIGURE 2 in simplified form.

at l the one transistor of the premises.

It is of the PNP type and comprises an emitter 2, a base 3, and acollector 4.

There are two circuits controlling, respectively, conductivity andnon-conductivity of'the transistor it; ener gized with unidirectionalpotential of opposite polarity, from quadrilateral rectifiers 5 and 6,which receive input from alternating supply mains 7-5. p 1

As to one of said circuits, a half wave of current from main 7 has apath by wire 9, through rectifier 6, by wire 10, through normally opencontacts of one contactor 11, by wires 12 andM to emitter 2; and throughthe activating circuit of the transistor to base 3, and thence by wires15 and 16 and a resistor R1, through the rectifier 6 and by wire 17 tosupply line 8.

The next half wave has a similar path from main 8 by wire 17, throughrectifier 6, and thence by wire 16), contactor 11, and again by wire 1 3through" the emitter 2.

and base 3 of the transistor and back by wires 15-16 and resistor R1,through the rectifier 6 to wire 9 and main 7.

Upon closing contactor ill, full wave rectified current in this circuitwill go from emitter 2 to base 3 in the transistor and will render thetransistor conducting, as is well known.

As to the other of said circuits, a half wave of current from supplymain 7 has a path by wire 13, through the rectifier 5, resistor R2,wires lid and 15, to the base 3 and from emitter 2 by wires 1 12 and 19;through normally I open contacts of another contactor 20, wire 21through the rectifier 5, and by wire 21A to line 8. I

Similarly the next half Wave will go from main 6 by wire 21A throughrectifier 5, by resistor R2, wires 16-115 to base 3 of the transistorand from its emitter 2,'ancl by wires 14-12, contactor 2t? and rectifier5 and wire 18 to main 7.

Upon closing contactor 2h, full wave rectified current in this circuitwill go from 3 to 2. in the transistor and will render the transistornon-conducting.

It the contactor 11 happens to be operated when contactor 2% isoperated, the current in the line of the contactor 2t), flowing in thetransistor from 3 to 2, will overpower the current in line of closedcontactor 11 and I render the transistor non-conducting.

To insure that this overpowering effect will occur, and assuming equaloutput potentials for the rectifiers 5 and 6, the resistance of resistorR2 will be made less than that of R1.

nected across the mains 7 and h; and having a secondary 25 with amidpoint 26, from which a wire 27 goes to I the said wire 14 and thenceto the emitter 2 of the transistor.

A load 28 to be energized is connected in a line 2-29 between thecollector 4 and the mid-point Sit, of the primary 31, of a feedbacktransformer $2.

The ends of the primary 31 are connected, respectively, by wires 33-34through rectifier units 3 36, to the ends of the main transformersecondary 25.

The secondary 37 of the transformer 32 has a mid-point 38 connected by awire 39 to the wire 15 and thus to the base 3.

The ends of the secondary 37 are connected, respectively, throughrectifier units, 4t} and 41, to a connection point 42, and thencethrough a resistor R3 to the Wire 27.

7 At the main transformer 24 the halves of the secondary 25 aredesignated as 43 and 44.

At the feedback transformer 32, the halves of the primary 31 aredesignated as 45 and 46; and the halves of the secondary 37 aredesignated as 47 and 48.

In the operation of the system of FIGURE 1, all current to energize theload 28 must go through the transistor; and normally, contactor 11 andcontactor 20 are open; and the transistor 1 is non-conducting and nocurrent fiows to the load. 7

Upon closing contactor 11, current flows over the above describedcircuit from line 8, by way of wire 17, rectifier 6, wire 1%, inductor22, contactor 11, wire Hand wire 14 into the emitter 2 and from the base3 by way of wire 15, wirelo, resistor R1, rectifier 6 and wire 9 to line'7; and this renders the transistor 1 conducting.

Current then flows from the mid-point 26 of the main transformer 24- bywires 27and 14 to the emitter 2 and out at the collector 4-, and by wire29 through the load 28, to the mid-point as of the primary 31 of thefeedback transformer 32; and thence back to the main transformersecondary 25. v

In fuller explanation of this load energizing circuit; a

'half wave of alternating current in the main secondary in only the halfMs of the secondary to the wire 27; and; at the feedback transformer,the how from the mid-point 39 will be through rectifier 35 and back tothe half 44.

Both half waves from the main transformer 24 thus flow downwardly asunidirectional current through the transistor l and the load 28, uponrendering the transistor conducting by closing contactor illl.

As explained in the premises it is a part of the invention that thetransistor will be maintained conducting after opening the contactor it,that is upon only momentary closing thereof. This is provided for asfollows.

At the feedback transformer 32, the primary halves 45 and 46 are, asdescribed, alternately energized in correspondence with the half wavesof the alternating current supply. They accordingly induce potential inthe sec ondary 37 alternately in opposite directions.

Potential in the up direction, corresponding to one half wave of thesupply, can produce current in only the upper secondary half 48, currentflow in the half 47 being blocked by the rectifier 41, and the currentthus produced flows up in the half 48, through rectifier 40, to point42, through resistor'Rs to wire 27 and thence by wire it, in at theemitter 2 of the transistor, out at its base 3, and by wire 15 to wire39 and thence back to the mid-point S8 of the secondary.

Similarlyas will now be understood, potential in the down direction insecondary 37 can produce current in only the lower secondary half 47,which will fiow downwardly, through the rectifier 41 and resistor R3 towires 27 and M, and thence as before.

Thus corresponding to each half wave of the alternating current supplythere is an impulse of unidirectional current going into the transistorat its emitter 2 and out'at its base 3, which will maintain itsconducting. Such current, in the absence of countervailing provisionswould go to zero value between each impulse and the next one, and toprevent this, an inductor 49 is placed in the line of the wire 27,through which the impulses must flow successively. The action of such aninductor to convert the half wave unidirectional impulses intocontinuous unidirectional current has'been explained in connection withdependently of the circuit controlled by the contactor 11, and thiscontactor may then be restored.

It will thus be seen that only an impulse of current through thecontactor 11 is needed, to set the system in condition to supplyunidirectional full current to the load 28 from the alternating currentsupply mains 7-8.

To cut off the current to the load 28, or reduce it to zero value as ofthe premises the other contactor 20-which is normally in restored oropen condition is operated to close its contacts.

It will first be assumed that the contactor 11 has been restored, or itscontacts opened; and that the transistor is being maintained conductingby the load feedback transformer 32 as described; this being thecondition for the mode of operation here described.

Contacts 20 being operated, current flows from the supply line 7 overthe above described path, wire 18, rectifier 5, resistor R2, wire 16,wire 15, to the base 3, and out at emitter 2, and by Wire 14', Wire 12and Wire 19, through contactor 2'0, and by wire 21, rectifier 5 and wire21A to supply line 8.

This current in the transistor, as is well known, is in the direction torender the transistor non-conducting, and it may be made strong enoughto do so and to overcome the conductivity maintaining current from thefeedback transformer 32 above described, by giving a suitably low valueto the resistor R2; and the flow of load current through the transistorstops.

Momentary closure of contactor 20, that is, a pulse only of currenttherethrough, is suflicient to produce this result and contactor 20 maynow be restored, and the conditions of the system then obtaining willremain unchanged.

As a second mode of operation, the contactor 11 may be operated to setthe system to supply full current to the load 28 as described, andthereafter be maintained operated. Then to reduce the load current tozero, the contactor 20 may be operated or its contacts closed.

As will be apparent from the foregoing description, current through thecontactor 11 is in the direction to render the transistor conducting,and current through the contactor 20 is in the opposite direction, torender it non-conducting.

When contactor 20 is closed and contactor 11 is being maintained closed,the opposing current through contactor 20 is made great enough tooverpower both the current through contactor 11 and that from thefeedback transformer 32 and render the transistor non-conducting, andstop the load current.

This can be effected by providing the resistor R2 in the path of thecontactor 20, with sufliciently low resistance.

Upon again opening or restoring contactor 20, contactor 11 will still bemaintained operated and the current therethrough will render thetransistor conducting and the load current will again flow through it.

Thus by this second mode of operation, the load current can be reducedto zero by operating the contactor 20, and raised again to its fullvalue by restoring the contactor 20, and so on alternately.

The difference in this mode of operation is that when the contactor 20is closed, it must be maintained closed as long as zero load current iswanted, and not be closed only momentarily.

A modification of the above described system is illustrated in FIGURE 2,and explained by the saturation curve of FIGURE 3.

In FIGURE 2, some of the parts are the same as in FIGURE 1 and have beengiven the same reference numerals to identify them.

At 50 is a closed magnetic circuit which functions in some respects likea transformer core, having a secondary winding 51 and two primarywindings 52 and 53 thereon.

The secondary winding 51 has one end connected by wire 12 to a point 13and thence by wire -14 to the emitter 2 of the transistor 1; and has theother end connected by wires 16 and to the transistor base 3.

The primary winding 52 is in a circuit connected to the alternatingcurrent supply mains 7-8. The circuit provides a current path througlrawire 54 from the main 8, through a quadrilateral rectifier 55 to a wire56 thence through the winding 52 and through a normally open or restoredcontactor 57, when closed, and through an inductor 58 through therectifier 55, and by wire 59 to the main 7.

The primary winding 53 is in a similar circuit, providing a current paththrough a wire 60 from the main 7, through -a rectifier 6-1 to a wire 62thence through an inductor 63 and through a normally open or restoredcontactor 64, when closed, and-through the winding 53 to a Wire 65 andthrough the rectifier 61 and by a wire 66 to the main 8.

The primary windings 52 and 53 are wound and connected so that theirmagnetomotive forces will be in opposite directions, and preferablyequal.

The material of the magnetic core 50 is chosen to have a hysteresis loopwith characteristics such as illustrated in FIGURE 3. Magnetomotiveforce in the positive, or negative, direction (right or left) from thepoint 67, Will effect a rapid rise, or fall, of the flux to saturation,as at 68 or 69; The saturation value of flux remains substantially levelwhen the magnetomotive'force is removed, giving a high value of residualflux as at the points 70'or 71.

In a first mode of operation of the system of FIGURE 2 the .contactors57 and 64 are both normally open.

To cause the system to supply full current to the load by wire 29, thecontactor 57 is operated or closed. Current from the lines 8 and 7,rectified at the rectifier 55 gives current to the primary winding 52over the above described circuit.

Assuming that the core 50 has been saturated by previous operation andthat its residual saturation flux is at the point 71 of FIGURE 3, theampere turns of the'prirnary winding 52, will go, from point 67 to point72 in FIGURE 3, and will reverse the flux in the core 50 and raise it tosaturation at the point 73.

This change of flux in the core 50 will generate an impulse of currentin the secondary winding 51; the windings 52 and 51 being poled so thatthe current impulse flows upwardly in the secondary 5'1 and out on lines12 and 14 to the emitter 2 of the transistor and out therefrom at thebase 3 and back by line 16 to the secondary 51.

The transistor 1 is thereby rendered conducting, and as described forFIGURE 1 full load current flows in lines 27 and 14 through thetransistor and by wire 29 to the load. The contactor 57 is then openedand the conductivity of the transistor is sustained by flow of currentas in FIGURE 1, from the feedback transformer 32 along wires 27 and 14,in at the emitter 2 and out at the base 3, and back to the feedbacktransformer 32 by wire 39.

Upon opening of contactor 57, de-energizing primary 52, the flux, FIGURE3, goes back from point 73 to point 70 and remains there as residualflux.

The saturation line 68 being almost level, very little change of fluxand negligible impulse in winding 51 occurs.

To reduce the load current to zero, contactor 64 is operated closing itscontacts and by the above described circuit, energizes primary winding53, the ampere turns thereof going, say, from point 67 to point 74FIGURE 3.

Winding 53 being poled reversely with respect to winding 52, this givesa quick change of flux, in FIGURE 3, from point 70 to point 75; and thisquick change of flux generates an impulse of current in winding 51 inthe down direction, and it flows therefrom in wires 16-15 to base 3 andout at emitter 2 and by wires 14-12 to secondary 51.

This as will be understood renders the transistor nonconducting, andload current ceases to flow therethrough.

Contactor 64 may now be opened or restored, and the flux will go topoint 71 as residual flux; but again, due to the almost levelcharacteristic of the saturation line 69,

T? little change of flux value and a negligible impulse in winding 51occurs.

Thus in FIGURE 1 by a first mode of operation, the contactor 11 isclosed to give full load current, and is then opened; and the contactor20 is closed to give zero load current, and then opened; and by a secondmode of operation contactor 11 is closed to give full load current andis held closed, and contactor 269 is closed and opened successively togive zero and full load currentsuccessively.

The first mode of operation of FIGURE 1 has also been described abovemodified by the arrangement in FIG- URE 2, with primary windings 52 and53 of opposite magnetomotive force, and preferably equal.

It now the second mode of operation as well as the first mode werewanted with FIGURE 1 modified by FIGURE 2, then a primary winding 53would be provided of greater magnetomotive force than that of thewinding 52, say twice as much. Then for the first mode of operation,when winding 53 is energized alone by contactor 64, to give zero loadcurrent, the contactor 57 being open, the first said mode of operationwould be performed with the diflerence that in FIGURE 3, the point '74would be twice as far from the point 67 as it is in FIGURE 3, and thepoint 75 would be twice as far along on the saturation curve 69 as thepoint 7-5; but the change in the fiux pro ducing. an impulse in winding51 would be that from point 70 to point 75 because the flux would changevery little beyond the point '75; and the impulse would be littledifferent from that above described for the first mode of operation withequal windings 53 and 52.

For the second mode of operation, to give zero current to the load byclosing cont actor 64 with contactor 57 held closed, the flux at thetime would be say, at the point 73 for the magnetornotive force of thewinding 52 alone, at point 72.

Upon closing contactor 64 giving the double magneto motive force ofwinding 53, the latter would predominate over that of winding 52 andshift the resultant magneto motive force from point 72 to point '74 andthe flux at. the point 73 would go to point 75 and produce the sameimpulse as in the first mode of operation, and upon successively closingand opening the contactor 64, with contactor 57 closed, the flux wouldgo back and forth between points 75 and 73 and give successivelyopposite impulses in the secondary 51 and cause the load current tochange from zero to full value successively.

It is convenient for purposes'of the claims, to refer to the circuit inthe transistor from the terminal end of the emitter 2 to the terminalside of the base 3, as the transistor activating circuit or path, torender the transistor conducting or nonconducting according to thedirection of current in said path.

In FIGURE 4 is shown a system of the general type of FIGURE 2 butsimplified and having a third mode of operation. It is arranged as anacross-the-line class of diagrams.

Some of the parts have the same reference characters as in FIGURE 2 toidentify them without further description.

The saturable transformer 50 has its secondary 51 always connected as inFIGURE 2 to the emitter 2 of the transistor l by wires 12 and 14 and tothe base 3 by wires 16 and E5. The conductivity maintaining circuit isshown at wire 27, wire 14, wire 15 and wire 39.

A primary 76 is always connected across alternating current mains 7 and8 through a rectifier 77 and is therefore always energized with halfwaves of the same polarity say positive. t

A primary '7 8 is connected across the mains 7-8 through a rectifier 79and through a contactor 84 which is normally closed; so that normallythe primary '78 is energized with alternating current half waves whichare synchronous with the half waves in the primary 7s.

The primaries 7'6 and 73 are connected so that their magnetomotiveforces are in opposition; and the ma ensasor g netomotive force of theprimary 78 is greater than that of the primary 7%, say twice as great.

Normally, when potential is first supplied to the lines 7-8, bothprimaries 76 and 78 are simultaneously energized, and the primary "i6overpowersthe primary 7e and saturates the transformer, as indicated,for example by the saturationline as of FIGURE 3.

Any impulse generated in the secondary 51 by the primary '78 as the fluxrises, goes downwardly in the secondary Stand in at the base 3 and outat the emitter 2; the secondary 51 being connected so that this will bethe'case; and the transistor will be non-conducting and no load currentwill flow through it.

In operation, to give current to the load 28, the contactor 8i) isopened. This de-energizes primary 78, leaving the primary 76 energizedalone.

The flux in FIGURE 3 then goes from the saturation line 69, through thepoint 71 and abruptly to the saturation line as, under themagnetornotive force of primary 76 as represented say by the point 72.

This rapid change of fiux generates current in the secondary 51 in thereverse direction, upwardly, the current going in at the emitter 2 andout at the base 3 and rendering the transistor conducting; and load.current flows through it and through the load 2% by wire 27, wire 14 andwire 2%.

This is an impulse, and at its end, Whn it comes to zero, the transistorwould become non-conducting and cut oil the load current, but the loadcurrent, by means of the feedback current already described, immediatelyflows in the wire 14, wire 15 and wire 39 and keeps it conducting.

To reduce the load current to zero, contactor 80 is closed, whichenergizes the primary 78 at its said preponderating value, which asdescribed renders the transistor non-conducting.

In this form, FIGURE 4, as another mode of 0peration, with load currentflowing, and maintaining the transistor conducting, the load current canbe cut oil or reduced to zero by opening the feedback maintainingcircuit, for example, by momentarily opening a contactor 81 in the lineof the wire 39 I claim as my invention: a

1. In an electric control system, a transistor having a main and anactivating circuit, a main source of current connected to a load; themain circuit or said transistor connected between the load and thesource to allow current flow therethrough when conductive and renderedconducting; a saturable magnetic core. having a firstand second primarywinding and a secondary winding; said secondary winding connected to theacti vating circuit of said transistor; a first circuit means comprisinga first contactor, said first primary winding and a direct currentsource connected in circuit, and upon closure of the first contactorcurrent flows through said first primary winding to saturate the core inone direction and induce a pulse in the secondary winding to supply apulse of current to the activating circuit of said transistor to renderit conducting; a second circuit means comprising a second contactor,said second primary winding and a direct current source connected incircuit, and upon closure of the second contactor current flows throughsaid second primary winding to saturate the core in the oppositedirection and induce a pulse in the secondary winding to supply a pulseof current to the activating circuit of said transistor in the directionto render said transistor non-conducting. 7 V

2. A control system as described in claim 1 and in which the maincurrent source is a main transformer having a primary and a secondarywinding; said main primary winding connected to a pair of alternatingcurrent means, and one side of the load is connected through the maincircuit of said transistor to a mid-point 'of the main 9 transformersecondary winding; and the other side of the load is connected to themid-point of a primary winding of a feedback transformer, the oppositeends of said primary winding of the feedback transformer are connectedto the opposite ends of the main transformer secondary winding; and thesustaining circuit is connected to a secondary winding of the feedbacktransformer and is supplied with current therefrom.

3. In an electric control system, a transistor having a main and anactivating circuit, a main source of current connected to a load; themain circuit of said transistor connected between the load and thesource to allow current flow therethrough when conductive and toprohibit said current flow when non-conductive; a sustaining circuitarranged to drive current from the load current and connected to theactivating circuit of said transistor to maintain it conducting whenonce it has been rendered conducting; a saturable magnetic core having afirst and second primary Winding and a secondary winding, and having theproperty of maintaining residual flux at the saturation value when theprimary winding saturating it is at least momentarily energized; saidfirst and second primary windings poled to develop magnetomotive forcesin pposite directions to saturate the core in opposite directions; saidsecondary winding having its opposite ends connected across theactivating circuit of said transistor; a first circuit means comprisinga first contactor, said first primary winding and a direct currentsource connected in circuit, and upon closure of the first contact orcurrent flows through said first primary winding to saturate the core inone direction and induce a pulse in the secondary winding to supply apulse of current to the activating circuit of said transistor to renderit conducting; and a second circuit means comprising a second contactor,said second primary winding and a direct current source connected incircuit, and upon closure of the second con- I tactor current flowsthrough said second primary winding 10 to saturate the core in theopposite direction and induce a pulse in the secondary winding to supplya pulse of current to the activating circuit of said transistor in thedirection to render said transistor non-conducting.

4. An electric control system as described in claim 3 and in which theopposite magnetomotive forces are substantially equal.

5. An electric control system as described in claim 3 and in which onemagnetomotive force exceeds the other sufficiently to reverse thesaturating flux in the core being produced by the other, and to saturatethe core in the opposite direction.

6. An electric control system as described in claim 3 and in which themain current source is a main transformer having a primary and asecondary winding; said main transformer primary winding connected toapair of alternating current means, and one side of the load is connectedthrough the main circuit of said transistor to a midpoint of the maintransformer secondary winding; and the other side of the load isconnected to the mid-point of a primary winding of a feedbacktransformer, the opposite ends of said primary winding of the feedbacktransformer being connected to the opposite ends of the main transformersecondary winding; and the sustaining circuit is connected to asecondary winding of the feedback transformer and is supplied withcurrent therefrom.

References Cited by the Examiner UNITED STATES PATENTS LLOYD McCOLLUM,Primary Examiner.

1. IN AN ELECTRIC CONTROL SYSTEM, A TRANSISTOR HAVING A MAIN AND ANACTIVATING CIRCUIT, A MAIN SOURCE OF CURRENT CONNECTED TO A LOAD; THEMAIN CIRCUIT OF SAID TRAN SISTOR CONNECTED BETWEEN THE LOAD AND THESOURCE TO ALLOW CURRENT FLOW THERETHROUGH WHEN THE CONDUCTIVE; A TOPROHIBIT SAID CURRENT FLOW WHEN NON-CONDUCTIVE; A SUSTAINING CIRCUITARRANGED TO DRIVE CURRENT FROM THE LOAD CURRENT AND CONNECTED TO THEACTIVATING CIRCUIT OF SAID TRANSISTOR TO MAINTAIN IT CONDUCTING WHENONCE IT HAS BEEN RENDERED CONDUCTING; A SATURABLE MAGNETIC CORE HAVING AFIRST AND SECOND PRIMARY WINDING AND A SECONDARY WINDING; SAID SECONDARYWINDING CONNECTED TO THE ACTIVATING CIRCUIT OF SAID TRANSISTOR; A FIRSTCIRCUIT MEANS COMPRISING A FIRST CONTACTOR, SAID FIRST PRIMARY WINDINGAND A DIRECT CURRENT SOURCE CONNECTED IN CIRCUIT, AND UPON CLOSURE OFTHE FIRST CONTACTOR CURRENT FLOWS THROUGH SAID FIRST PRIMARY WINDING TOSATURATE THE CORE IN ONE DIRECTION AND INDUCE A PULSE IN THE SECONDARYWINDING TO SUPPLY A PULSE OF CURRENT TO THE ACTIVATING CIRCUIT OF SAIDTRANSISTOR TO RENDER IT CONDUCTING; A SECOND CIRCUIT MEANS COMPRISING ASECOND CONTACTOR, SAID SECONDARY PRIMARY WINDING AND A DIRECT CURRENTSOURCE CONNECTED IN CIRCUIT, AND UPON CLOSURE OF THE SECOND CONTACTORCURRENT FLOWS THROUGH SAID SECOND PRIMARY WINDING TO SATURATE THE COREIN THE OPPOSITE DIRECTION AND INDUCE A PULSE IN THE SECONDARY WINDING TOSUPPLY A PULSE OF CURRENT TO THE ACTIVATING CIRCUIT OF SAID TRANSISTORIN THE DIRECTION TO RENDER SAID TRANSISTOR NON-CONDUCTING.